Effects of Pseudomonas fluorescens on the growth of Listeria monocytogenes and Listeria innocua in skimmed milk

Listeria monocytogenes and L. innocua (single strains isolated from a local milk-processing plant) were cultured alone, in combination, or in combination with Pseudomonas fluorescens ATCC 13525, at 7 degrees C or 14 degrees C in 10 % skimmed milk. Culture pH and bacterial population densities were monitored over 48 days. Pure cultures of L. monocytogenes or L. innocua showed a sharp decline in pH from about day 20 onwards at 7 degrees C, and from about day 8-12 onwards at 14 degrees C. In mixed cultures with P. fluorescens, the pH decline started earlier (on about day 4 at 7 degrees C, and about day 1 at 14 degrees C), and counts of the Listeria species began to decline from about day 12 onwards (7 degrees C) or day 4 onwards (14 degrees C). Neither of the Listeria species had noticeable effects on growth of P. fluorescens.